/Engine/Source/Runtime/AIModule/Classes/Navigation/CrowdManager.h
Called by the nav system when a navigation data instance is removed. The crowd manager will see if it's the nav data being used by it an if so try to find another one. If there's none the crowd manager will stop working.
virtual void OnNavDataUnregistered ( [ANavigationData](API\Runtime\NavigationSystem\ANavigationData) & NavData )